The invention relates to a process and a circuit arrangement for decoding the output signal of a camera tube in a single-tube color television camera with two color strip filters which are rotated towards the picture axis, where, in the output signal of the camera tube the low-frequency spectrum of a luminance signal is separated from the color carrier frequency band range of color component signals which are intermeshed in a so-called quarter line offset, and where the color component signals are obtained via comb filters.
A process of this type, and a circuit arrangement for the execution of this process are known and described in the British Pat. No. 1,333,672. The output signal of the camera tube contains a luminance signal in the spectrum of which spectral lines occur as whole-numbered multiples of the line frequency. In addition the output signal of the camera tube contains two color signals whose spectra are intermeshed with one another and with the spectral lines of the luminance signal and in fact normally in accordance with the PAL-standard in a so-called quarter-line offset. The spectral lines of the two color spectra here occur fundamentally in a frequency band around the color carrier frequency which is established by the repetition frequency of the color strips. In accordance with the prior art, the three intermeshed spectra are separated in that the output signal of the camera tube is fed to a low-pass filter and to a band-pass filter. The low-pass filter limits the luminance signal range to a low-frequency range up to a lower cut-off frequency at which the color carrier frequency band range, which is to be processed, of the color signals, commences. The band-pass filter limits this range from the aforementioned lower cut-off frequency to an upper cut-off frequency which is symmetrical to the former in respect of the color carrier frequency. The spectral lines lying above the upper cut-off frequency of the band-pass filter are not processed. The intermeshed color spectra which are obtained from the band-pass filter are then processed via comb filters and demodulators to form two color component signals. The third color component signal is obtained by combining the first two with the output signal of the low-pass filter, the luminance signal (see in particular FIGS. 3 and 6 and British Pat. No. 1,333,672).
This known decoding of the output signal of the camera tube of a single-tube color television camera on the one hand has the disadvantage that in order to obtain the luminance signal, the spectrum of the spectral lines containing the luminance information must be drastically limited to the range from zero to the lower cut-off frequency. The information contained in the spectral lines above this cut-off frequency is lost for the luminance signal. This means that the luminance signal becomes relatively poorly defined. On the other hand, separating the color spectra from one another via simple comb filters has the disadvantage that the acquired color component signals possess a relatively poor resolution in the vertical direction, i.e., that horizontal lines become blurred. These color component signals with a poor vertical resolution are combined with the luminance signal from the low-pass filter, which latter possesses no such impairment of the vertical resolution. The third color component signal obtained by the combination has a better vertical resolution than the two other color component signals. This means that horizontal colored edges occur. The impairment of the vertical resolution by the comb filter occurs due to the comb filter characteristic, in accordance with which those frequencies located between the accumulation points, i.e., the transmissive points of the comb filter, are suppressed. For example, maximum contrast values, with which alternating differing extreme values with half the line frequency occur line by line, are no longer transmitted.